The present invention relates to means for locating a workpiece fixture on a table during movement thereof from one machining position to another and in particular to a dual centering pin apparatus which selectively engages one or more slots and openings on the downwardly facing surface of the fixture so as to provide controlled translation and rotation thereof.
In standard machining practice, the machining of workpieces of any substantial size involves time consuming, laborious repositioning of the workpiece as various regions of the work are to be machined. Often, it becomes necessary to use hoists and other power devices for elevating and moving the workpiece about and for lowering the workpiece into the proper position for the machining of respective regions of the workpiece. Additionally, precise positioning of the workpiece in a selected position under such conditions is difficult and cannot always be achieved with the desired accuracy.
In order to overcome these problems, an air float table wherein the workpiece is supported on a film of pressurized air has been developed. Such a table, which is described in the aforementioned copending patent application Ser. No. 684,725, is provided with fluid passages and a plurality of fluid outlets distributed over the surface of the table so that a cushion of pressurized air may be provided underneath the workpiece fixture. By virtue of the fluid pressure film, substantially friction free movement of the fixture on the table is possible thereby permitting positioning and repositioning to be accomplished by a single operator without the need for extraneous hoisting equipment.
In order for the fixture to be rotated and translated from one position to another, the table may be provided with a main pivot pin which projects upwardly from the table surface. The pin may be receivable in a socket in the bottom of the fixture in which case the fixture is constrained to move circularly on the table. Alternatively, the socket may be replaced by a slot so that the fixture is not only rotatable on the bed but is translatable thereon in various desired directions. Cooperating elements of retractable pin and socket locating devices on the fixture and table provide for location of the fixture in predetermined positions on the table. Clamps are also provided to clamp the fixture in the located positions on the table during machining.
Although an air float system of this type is advantageous in repositioning heavy workpieces, there are certain attendant dangers which result from the substantially friction free relationship between the workpiece fixture and the table surface. For example, the workpiece and fixture can slide off the table if the operator does not exercise extreme caution during repositioning to assure that he has it fully under control. The tremendous masses of large workpieces and the high momenta which result when they are moved, however, make it difficult to stop the workpiece manually. Obviously, the potential for injury to the operator and damage to the workpiece and machine is very great.
Another problem is that of accurately stopping the workpiece at the desired position for subsequent engagement of the locating pins and sockets. The operator must therefore search for the desired position through repeated trial and error thereby resulting in a loss of productive machining time. In the case where the fixture is provided with a pair of crossed slots for dual translation, it is difficult for the operator to determine when the centering pin is located at the intersection of the two slots so that movement from one slot to the other can be accomplished. Accurately located rotation centers at various locations within the slots is also desirable for multiple machining operations.